1. Field of the Invention
This invention relates generally to a semiconductor technique and more particularly to a film with a low dielectric constant (referred to as xe2x80x9clow-kxe2x80x9d) formed on a semiconductor substrate by using a plasma CVD (chemical vapor deposition) apparatus.
2. Description of the Related Art
Because of the recent rise in requirements for the large-scale integration of semiconductor devices, a multi-layered wiring technique attracts a great deal of attention. In these multi-layered structures, however, capacitance among individual wires hinders high speed operations. In order to reduce the capacitance it is necessary to reduce dielectric constant of the dielectric film. Thus, various materials having a relatively low dielectric constant have been developed for insulation films. Further, a hard mask such as SiN is used in formation of semiconductor structure, but such a film has a dielectric constant of around 8. It is also preferable for hard masks to have a low dielectric constant.
A reduction of dielectric constant can be achieved by methods disclosed in U.S. Pat. Nos. 6,352,945, 6,383,955, 6,410,463, and 6,432,846, all of which are incorporated herein by reference in their entirety. In these methods, siloxan polymers are formed from organo-silicon reactant materials in a plasma CVD apparatus wherein the residence time of a reaction gas flowing through a reaction chamber is lengthened. Although the films composed of siloxan polymers deposited by the methods show low dielectric constants and reasonable mechanical properties as compared with films composed of conventional silicon oxide films, hardness of the films is not targeted in the above U.S. patents.
One aspect of this invention involves a method for forming a film having a low dielectric constant and high mechanical hardness on a semiconductor substrate by plasma reaction. In an embodiment, a method comprises the steps of: (i) introducing a silicon-containing hydrocarbon gas as a source gas into a reaction space for plasma CVD processing wherein a semiconductor substrate is placed, said silicon-containing hydrocarbon being capable of self-polymerization or polymerization with cross-linkers to form siloxan polymers by plasma reaction; and (ii) applying radio-frequency (RF) power of 1,000 W or higher (e.g., 1,000-20,000 W, preferably 2,000-15,000 W, and in an embodiment, 3,000-10,000 W) to the reaction space while maintaining a pressure of the reaction space at 100 Pa or higher to activate plasma polymerization reaction in the reaction space, thereby forming a thin film on the semiconductor substrate. According to the embodiment, a film having a low dielectric constant of 2.5-3.2, preferably 2.8-3.1, can have as high hardness as 1.0-5.0 GPa, preferably 1.5-3.5 GPa.
In another embodiment, a method further comprises introducing an inert gas such as Ar, Ne, and He into the reaction space to maintain the pressure during the plasma polymerization reaction. The flow of inert gas may be 5-200% of the source gas, preferably 10-120% (including 20%, 40%, 60%, 80%, 100%, and a range including any two of the foregoing).
In an embodiment, the: pressure may be 300-1,000 Pa (including 400 Pa, 500 Pa, 600 Pa, 700 Pa, 800 Pa, and a range including any two of the foregoing). For plasma polymerization, relatively high pressure is preferable. Further, RF power may be 1,000-20,000 W (including 1,500 W, 2,000 W, 2,500 W, 3,000 W, 3,500 W, 4,000 W, 5,000 W, 6,000 W, 7,000 W, 8,000 W, 9,000 W, 10,000 W, 12,000 W, and 15,000 W, and a range including any two of the foregoing). The above may apply to a substrate having a diameter of 300 mm, although any suitable size substrate can be treated. High RF power can be expressed by power density (W/cm2). In the present invention, the RF power density may be in the range of 1.4 to 28 W/cm2, preferably, 2.8 to 21 W/cm2, and in an embodiment, 4.2 to 14 W/cm2). The density can be calculated by dividing the total intensity (W) by the area of a substrate (cm2). For example, if a substrate having a diameter of 300 mm is used, the area is approximately 707 cm2, and 2,000 W is 2.8 W/cm2.
Further, the RF power can be applied by overlaying high-frequency power and low-frequency power. Overlaying two frequencies is effective in promoting plasma polymerization. A high frequency may be 2 MHz or higher (including 5 MHz, 10 MHz, 15 MHz, 20 MHz, 25 MHz, and 30 MHz, e.g., 13.4_MHz or 27.12 MHz) and a low frequency may be lower than 2 MHz (including 1 MHz, 500 kHz, and 200 kHz, e.g., 400 kHz or 430kHz). The low frequency power may be 0% to 10% of the high frequency power (including 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, and a range of any two of the foregoing).
In an embodiment, no oxidizing gas may be added to the source gas for plasma polymerization reaction. When using a silicon-containing hydrocarbon having oxygen atoms in an amount effective to form a siloxan polymer structure, no oxidizing gas need to be added. The films deposited are not silicon oxide films. When adding an oxidizing gas, its flow may be less than 50% of the source gas, preferably 10%-30%.
In another example, a cross-linker agent such as alkanol and unsaturated hydrocarbon may be added as an additive gas, depending on the type of source gas (i.e., whether radical groups are not sufficient in the source gas compound itself). The flow of such cross-linkers, if any, may be 30%-200%, preferably 50%-150%.
As described above, in the present invention, a method comprises the steps of: (i) introducing a silicon-containing hydrocarbon gas as a source gas into a reaction space for plasma CVD processing; and (ii) applying radio-frequency (RF) power of 1,800 W or higher (preferably 2,000 W or higher) to the reaction space while maintaining a pressure of the reaction space at 100 Pa or higher (preferably 400 Pa or higher) to activate plasma polymerization reaction in the reaction space. In connection to high RF power application, U.S. Pat. No. 6,051,321 to Lee discloses the deposition of low-k films by plasma assisted CVD or transport polymerization techniques. In Lee, power levels in a range of 100 W to 4,000 W are used only when operating at very low pressures ranging from 0.01 mTorr to 10 mTorr (i.e., 0.0013 to 1.3 Pa) (column 19). These conditions relate to the technique of high density plasma CVD. Lee teaches that when the power is high, the pressure must be very low. In the present invention, the pressure is higher than Lee""s by the factor of 103-105. In the present invention, by applying high RF power at relatively high pressures, the hardness of a film can increase without significantly increasing the dielectric constant of the film composed of a siloxan polymer formed from silicon-containing hydrocarbon compounds.
The present invention can be adapted to any suitable film formation methods using various source gases, which include, but are not limited to, the methods disclosed in U.S. patent application No. 10/288,641 filed Nov. 5, 2002, U.S. patent application No. 09/827,616 filed Apr. 6, 2001, U.S. Pat. No. 6,352,945, U.S. Pat. No. 6,383,955, U.S. Pat. No. 6,410,463, and U.S. Pat. No. 6,432,846, all of which are incorporated herein by reference in their; entirety. The films deposited can be used as an insulation film, etch stop film, hard mask, cap film, etc.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
Further aspects, features and advantages of this invention will become apparent from the detailed description of the preferred embodiments which follow.